1. Field of the Invention
The present invention relates to an optical fiber characteristic measuring device which produces a pulse of light incident to an optical fiber as an object to be measured and measures the characteristic of the optical fiber according to the returning light emitted from the optical fiber.
2. Description of Related Art
FIG. 5 is a block diagram showing a structure of an example of a conventional optical fiber characteristic measuring device. A light of constant frequency of xcexd0 which is emitted from the light source 1 is incident on an incident port 4i of a first optical directional coupler 4. The first optical directional coupler 4 has an incident port 4i and two emitting ports 4t1 and 4t2. The first optical directional coupler 4 separates the light incident on the incident port 4i into 2 directions and emits the light from the two emitting ports 4t1 and 4t2.
The light which is emitted from the emitting port 4t1 of the first optical directional coupler 4 is incident on the light pulse generating device 5. The light pulse generating device 5 is specifically an electro-optical switch. The light pulse generating device 5 extracts the light pulse from the incident light by turning the switch on and off and emits the extracted light pulse.
The light pulse emitted from the light pulse generating device 5 is incident on a light amplifier 6. The light amplifier 6 amplifies the incident light pulse to a predetermined level and emits the amplified light pulse. The light pulse emitted from the light amplifier 6 is incident on an incident port 7i of an optical switch 7. The optical switch 7 has three ports such as an incident port 7i, an emitting/incident port 7ti, and an emitting port 7t, and emits the light pulse which is incident on the incident port 7i from the emitting/incident port 7ti. The optical switch 7 also emits the returning light which is incident on the emitting/incident port 7ti from the emitting port 7t. 
The emitting/incident port 7ti of the optical switch 7 is connected to an end 9a of the optical fiber 9 as an object to be measured via an optical connector 8. Therefore, the light pulse emitted from the emitting/incident port 7ti of the optical switch 7 is incident on an end 9a of the optical fiber 9 via an optical connector 8. The returning light which is emitted from the end 9a of the optical fiber 9 is incident again on the emitting/incident port 7ti of the optical switch 7, and is further emitted from the emitting port 7t of the optical switch 7.
The returning light emitted from the emitting port 7t of the optical switch 7 is incident to the incident port 10i1 of the second light directional coupler 10. The second light directional coupler 10 has two incident ports such as 10i1 and 10i2 and two emitting ports such as 10t1 and 10t2. To the incident port 10i2 of the second light directional coupler 10, the light (hereinafter called xe2x80x9creference lightxe2x80x9d) emitted from the emitting port 4t2 of the first light directional coupler 4 is incident. Consequently, the second light directional coupler 10 combines the wave of the returning light which is incident from the incident port 10i1 and the wave of reference light which is incident from the incident port 10i2. The second light directional coupler 10 further separates the combined light into two directions, and emits the lights from the two emitting ports 10t1 and 10t2.
Both of the combined lights emitted from the two emitting ports 10t1 and 10t2 of the second light directional coupler 10 are received by balance receiving photodiode PD11. The balance receiving photodiode PD11 converts the combined lights which is received to an electric signal (beat signal) and outputs the converted electric signal (beat signal). The beat signal which is output by the balance light receiving photodiode PD11 is input to an amplifier 12. The amplifier 12 amplifies the input beat signal to a predetermined level and sends the amplified beat signal to a mixer 13.
The mixer 13 mixes the beat signal sent from the amplifier 12 and an RF signal generated by a signal generating circuit 14, and outputs the mixed signal. A control circuit 15 controls the signal generating circuit 14 and determines the frequency xcexdr of the RF signal generated by the signal generating circuit 14. The frequency xcexdr of the RF signal is set to a value which is close to 10.8 GHz as a shifting amount by the Brillouin scattering.
A low pass filter 16 inputs the mixed signal which is output by the mixer 13, removes high frequency component which is included in the mixed signal which is input, passes only low frequency component, and outputs a difference signal which is a low frequency component. The amplifier 17 amplifies the difference signal which is output by the low pass filter 16 to a predetermined level, and outputs the amplified difference signal. The signal process section 18 inputs the difference signal which is output by the amplifier 17, performs various signal treatment on the inputted difference signa, and determines the characteristic of the optical fiber 9.
Next, the operation of the optical fiber characteristic measuring device is explained. The light with the frequency of xcexd0 emitted from the light source 1 is sent to the light pulse generating circuit 5 via the light directional coupler 4. Then, the light pulse generating circuit 5 extracts the light pulse with the frequency of xcexd0 from the light which is sent.
The light pulse emitted from the light pulse generating circuit 5 is incident on the end 9a of the optical fiber 9 via an optical amplifier 6, an optical switch 7, and an optical connector 8. When the incident light pulse is transmitted in the optical fiber 9, Brillouin scattering, Rayleigh scattering, and reflection occur at several points in the optical fiber 9, then the returning light including the Brillouin scattered light, Rayleigh scattered light, and reflected light return to the end 9a from such several points. The returning light is emitted from the end 9a. 
The returning light emitted from the end 9a of the optical fiber 9 and including the Brillouin scattered light is incident again on the emitting/incident port 7ti of the optical switch 7 via the optical connector 8, and is further emitted from the emitting port 7t. The returning light emitted from the emitting port 7t of the optical switch 7 and including the Brillouin scattered light is incident on the incident port 10i1 of the second light directional coupler 10. Additionally, to another incident port 10i2 of the second light directional coupler 10, the reference light emitted from the emitting port 4t2 of the first light directional coupler 4 with a frequency of xcexd0.
The second light directional coupler 10 mixes the wave of the Brillouin scattering light with frequency of xcexd0xc2x1xcexdB and the wave of reference light with frequency of xcexd0. Consequently, resonance occur because the frequencies of these lights are so close that interference is caused. The frequency of the resonance is represented as the difference between the frequency of Brillouin scattering light such as xcexd0xc2x1xcexdB and the frequency of the reference light such as xcexd0. Therefore the frequency of the resonance becomes xcexdB.
When the mixed light in which the resonance of which frequency is xcexdB occurs is received by the balance receiving photodiode PD11, the balance receiving photodiode PD11 outputs the beat signal having the resonance of which frequency is xcexdB. The beat signal which is output by the balance receiving photodiode PD11 and has the resonance of which frequency is xcexdB is input to the mixer 13 via the amplifier 12. An RF signal of which frequency is xcexdr which is generated by the signal generating circuit 14 is input into the mixer 13 together with the beat signal having the resonance of which frequency is xcexdB, and these signals are mixed. Here, the frequency xcexdr of the RF signal which is generated by the signal generating circuit 14 is set quite close to the frequency xcexdB in advance. Then, the beat signal and the RF signal interfere; thus the resonance occurs. The frequency of the resonance is represented by a difference between the frequency xcexdB of the beat signal and the frequency xcexdr of the RF signal such as xcexdBxe2x88x92xcexdr. The frequency xcexdr of the RF signal which is generated by the signal generating circuit 14 is set quite close to the frequency xcexdB of the resonance of the beat signal.
When the mixed signal in which the resonance of which frequency is xcexdBxe2x88x92xcexdr occurs is input to the low pass filter 16, the low pass filter 16 cuts the high frequency signal (signal of which the frequency is xcexdB or xcexdr) included in the mixed signal, and outputs the difference signal having only frequency xcexdBxe2x88x92xcexdr of the resonance as a low frequency signal. The signal processing section 18 measures the frequency of the difference signal. Additionally, the signal processing section 18 calculates the frequency xcexdB of the beat signal from the frequency xcexdBxe2x88x92xcexdr of the difference signal which is measured, and calculates the shifting amount xcexdB due to the Brillouin scattering. Furthermore, the signal processing section 18 determines the distortion amount in a predetermined point in the optical fiber 9 from the shifting amount xcexdB which is calculated.
For a balance receiving photodiode PD11, and amplifier 12, a mixer 13, and a signal generating circuit 14 of the above described optical fiber characteristic measuring device, components having frequency characteristic so as to correspond to high frequencies such as 10.8 GHz of shifting amount xcexdB due to the Brillouin scattering need be used; thus, the problem is that the cost for such components increases.
The present invention was made in consideration of the above problem and provides an optical fiber characteristic measuring device which can reduce the cost of the above mentioned circuits and the like.
The invention according of the first aspect 1 is an optical fiber characteristic measuring device comprising a coherent light supply device which supplies a coherent light with a second frequency which is almost equal to the frequency of coherent light with a first frequency and the frequency of returning light emitted from this optical fiber when the coherent light with first frequency is incident to the optical fiber as an object to be measured, a light pulse generating device which converts the coherent light with first frequency which is supplied by the coherent light supply device to light pulse and emits the light pulse which is converted, a wave mixing device which mixes the wave of returning light emitted from the optical fiber and the wave of the coherent light with second frequency supplied from the coherent light supply device when the light pulse emitted by the light pulse generating device is incident on the optical fiber as an object to be measured and emits the mixed light, a opto-electrical converting device which converts the mixed light emitted from the wave mixing device to an electrical signal and outputs the electrical signal which is converted, a processing device which calculates a shifting amount to the frequency of the returning light emitted from the optical fiber from the first frequency of coherent light which is incident to the optical fiber as an object to be measured according to the electric signal which is output from the opto-electrical converting device and determines characteristic of the optical fiber from the calculated shifting amount.
The invention of the second aspect is an optical fiber characteristic measuring device according to the first aspect, wherein the coherent light supply device has a driving device which can output more than two kinds of driving current and a light source which can alter the frequency of the coherent light which is emitted corresponding to the driving current which is output by the driving device.
The invention according to the third aspect is an optical fiber characteristic measuring device according to the second aspect, wherein the light source is a distributed-feedback laser diode.
The invention according to the fourth aspect is an optical fiber characteristic measuring device according to the second aspect, wherein the returning light emitted from the optical fiber is Brillouin scattered light.
According to the present invention, parts having the frequency characteristic for corresponding to low frequency component for an opto-electrical converting device (balance receiving photodiode PD11 in the present embodiment) and a processing section (balance receiving photodiode PD11, amplifier 12, mixer 13, signal generating circuit 14 in the present embodiment); thus, the cost of opto-electrical converting device and the processing section can be reduced.